Illumination devices for light microscopes with incident illumination comprise a light source, a field diaphragm and illumination optics with which the field diaphragm is to be imaged into the region of the object. Basically the object field should be uniformly illuminated and have a sharp edge. This may be achieved by way of imaging the completely illuminated field diaphragm as accurately as possible into the object plane. The light source is for example a halogen lamp or LED. The illumination beam path may be led through the objective of the microscope or run laterally next to the observation beam path of the microscope, without going through the objective.
In order to lead the illumination beam path through the objective of the microscope, the illumination beam path may for example be coupled in by way of a beam splitter and be brought to overlap with the observation beam path. Alternatively, a splitter plate may be provided, with which one may fix different channels for detection and illumination. Both variants permit an illumination with an illumination axis perpendicular to the object plane and thus in principle a sharp imaging of the field diaphragm in the object plane. These means however have the disadvantage that the light intensity is reduced in the observation beam path.
For this reason, the coupling-in of light is often effected via deflection means, e.g. a prism or mirror. This is arranged outside the observation beam path, in particular laterally of this, in order to prevent a shading of the observation beam path by the deflection means. DE-B 103 32 602 discloses a light microscope with which the illumination beam path is deflected via a deflection means, such that the illumination beam path passes through the objective in its edge region. The illumination light is incident on the objective as a beam bundle which is parallel to the optical axis of the objective and is basically imaged in its focal plane. The objective is corrected for the observation beam path with regard to imaging errors. Since the observation beam path normally does not pass through the whole objective surface, but only the centric region of the objective, it is corrected less well for the illumination beam path which passes through the edge region of the objective. The illumination light may thus be imaged with aberrations onto the object plane.
With other illumination devices, the illumination light passes through the objective, however, on account of the coupling-in from the side, the axis of the illumination beam path runs at an angle to the optical axis of the objective for reasons of space.
Microscopes with an illumination beam path which does not pass through the objective but runs laterally past the objective are for example described in US-A 2001/0010592, DE-A 195 23 712 or DE-A 195 37 868. In this case, the axis of the illumination beam path likewise does not run parallel, but at an angle of about 1° to 10° to the optical axis of the objective and of the observation beam path.
An illumination with an illumination axis which is oblique (instead of perpendicular) to the object plane has the disadvantage that the field diaphragm may not be imaged sharply into the object plane over its complete area or along the complete scope of the circular diaphragm opening, and moreover is perspectively distorted. Moreover, undesired chromatic effects may occur at the edges of the illumination field. This is perceived as “unclean” optics and thus as a quality defect. Specific technical disadvantages are present if a defined, sharply bordered illumination spot is necessary for diagnostic purposes for example, or one is to image optical structures such as e.g. grid, annular structures, rectangle/gap, into the object field in a sharp manner.
US-A 2001/0010592, DE-A 195 23 712 or DE-A 195 37 868 discloses different measures, e.g. rotatable deflection mirrors, with which the focal plane of the illumination device may be adapted to the variable focal width of an objective with a zoom system arranged thereafter. However, the problem outlined above also exists with these microscopes or illumination devices. It may however be avoided by way of selecting an as small as possible coupling-in angle relative to the optical axis of the objective. This however necessitates large distances and is mostly not possible for reasons of space.